For the purpose of adjusting a light amount in an infrared region or cutting an entire light amount in the region, infrared absorbing glass has hitherto been used. A Cu2+ component in the infrared absorbing glass is involved in an infrared absorption principle of the glass. Specifically, absorption in an infrared region caused by d-d transition of a Cu2+ ion is utilized. In general, fluorophosphate-based glass is used as such infrared absorbing glass, but the fluorophosphate-based glass has a problem of low water resistance making handling difficult.
Meanwhile, in use of the infrared absorbing glass, the glass itself becomes hot in response to an amount of absorbed light particularly in the case where condensed light needs to be absorbed by the glass or where the glass is exposed to light including an absorption component for a long time period. In the glass that has absorbed light or has been exposed to light, a temperature difference of about from 100° C. to 200° C. easily arises in an object in some cases. For this reason, for example, in the case where a thin film such as an anti-reflection film is provided on a surface of the infrared absorbing glass, an internal stress is often generated between the thin film and a substrate. When the stress is concentrated at an interface between the thin film and the substrate, the thin film (an optical deposition film such as an anti-reflection film) is liable to be peeled off or cracked.
In addition, in the case of an oscillatory optical member, it is also considered that the optical member is liable to be broken when the above-mentioned internal stress is applied in addition to an external stress accompanied by oscillation. In view of the foregoing, the glass to be used is required to have resistance to temperature changes in addition to heat resistance particularly in an application in which intense light is locally absorbed.
To cope with the problems, for example, PTL 1 proposes infrared absorbing glass that is constructed of silica, copper oxide, and a network modified oxide and is aimed at improving durability of the infrared absorbing glass.
Although the infrared absorbing glass of PTL 1 has an infrared absorption ability, its heat resistance and durability are still insufficient owing to a system in which the network modified oxide is introduced in a network structure formed of silica.